The present invention relates to a novel process for the preparation of 2,3,5-trimethyl-p-benzoquinone by oxidizing phenols by means of oxygen in the presence of a two-phase liquid reaction medium containing a catalyst mixture of copper chloride and additionally a transition metal halide selected from the group consisting of iron, chromium, manganese, cobalt, nickel, zinc and a halide of a rare earth element. Both 2,3,5-trimethylphenol and 2,3,6-trimethylphenol may be used as reactants in that process.
2,3,5-Trimethyl-p-benzoquinone is an intermediate which is used inter alia in the preparation of .alpha.-tocopherols (vitamin E).
The oxidation of trimethylphenols to 2,3,5-trimethyl-p-benzoquinone is known.
The use of inorganic oxidizing agents, including potassium permanganate, manganese dioxide and lead oxide, has been described, it being necessary in prior-known processes to use stoichiometric amounts of the oxidizing agent. The use of stoichiometric amounts of those expensive oxidizing agents causes high chemicals consumption and produces streams of waste which are polluted with the corresponding reduced metals and must be regenerated or disposed of at great expense.
Also known are catalytic processes in which the trimethylphenol oxidation is carried out in the presence of a metal catalyst using an oxygen-containing gas as the oxidizing agent. Conversion of those processes for commercial application, for example using a cobalt-salene complex catalyst, is complicated and expensive owing to the short life of the catalyst, since the addition of not inconsiderable amounts of fresh catalyst and the disposal or costly treatment of considerable amounts of discharge stream polluted with metals are necessary.
In EP 0 659 727, for example, tetraaza[14]annulene which contains a complex-bonded heavy metal ion is described as the oxygen-carrying catalyst. That catalyst complex is destroyed during the oxidation and is not recyclable, so that it is not suitable for commercial use.
In this connection, U.S. Pat. No. 3,796,732 describes the use of copper chloride as the catalyst for the reaction, wherein the operation is carried out in a homogeneous phase in the presence of an inert solvent such as DMF and there arises the problem of recovery of the catalyst, which can be solved technically only with a great expenditure.
In JP 17585/1978, an improvement in the yields is described using a catalyst system consisting of copper ions and halogen ions. Disadvantages of that process are that, in spite of good yields, the space-time yield is low and it is necessary to extract the catalyst using large amounts of water and to remove water in order to recycle the catalyst, and, not least, that residual water has a negative effect on the catalyst performance of the recycled catalyst.
In JP 93931/1975, halogens or halogenated compounds are added during the recycling in order to maintain the catalyst activity, but those compounds are used up rapidly under the reaction conditions and therefore must be supplemented regularly. That is expensive in terms of process technology and leads to markedly increased production costs.
A possible method of avoiding the problems of catalyst recycling while simultaneously maintaining catalyst activity is described in RU-2 039 037, in which the oxidation of trimethylphenol and structurally related compounds in the presence of a heterogeneous catalyst by means of oxygen or an oxygen-containing gas is disclosed.
A disadvantage of that process has proved to be the expensive preparation of the heterogeneous catalyst, which is obtained by applying a monovalent copper chloride in the presence of ammonium chloride and an alkali metal chloride to aluminum hydroxide as support in the presence of a defined amount of phosphoric acid.
According to EP 0 127 888, aqueous solutions of Li(CuCl.sub.3) in the presence of a high excess of the corresponding lithium halide are used as the oxidation catalyst. It has been found, however, that despite good yields, conversion of that process for commercial application is not advantageous because large excesses of expensive lithium halide must be used, the complex copper(II) catalyst must be expensively prepared before the reaction, and at least equivalent amounts of the catalyst, based on trimethylphenol, must be used to achieve good yields.
EP 0 167 153 describes the use of an aqueous catalyst solution consisting of Li(CuCl.sub.3) or corresponding copper(II) complexes in the presence of an excess of the corresponding lithium halide.
In EP 0 294 584 there is also described a process for the preparation of 2,3,5-trimethyl-p-benzoquinone in the presence of a catalyst consisting of copper(II) chloride and lithium chloride in a two-phase reaction medium consisting of water and a mixture of an aromatic hydrocarbon and a lower aliphatic alcohol having from 1 to 4 carbon atoms. The use of a complex organic solvent mixture, which must be recovered by distillation following the reaction, is not advantageous from a commercial point of view.
Another variant of the oxidation in a two-phase reaction system is described in EP 0 369 824. The catalyst consists of a binary system consisting of a copper(II) halide and a nitrogen-containing compound, preferably a hydroxylamine, an oxime or an amine or the corresponding ammonium salts. It has proved disadvantageous that the nitrogen-containing catalyst component is decomposed under oxidative conditions, cannot be recycled, and therefore gives rise to high costs.
EP 0 475 272 describes oxidation in the presence of an oxygen-containing gas using a catalyst consisting of a copper(II) halide and an alkaline earth metal halide in a two-phase solvent system consisting of water and a saturated aliphatic alcohol having from 5 to 10 carbon atoms. In that process, the active catalyst is formed in situ from the copper(II) salts and the alkaline earth metal additives and the organic solvent system has a sufficiently high flash point in comparison with the reaction temperatures used. However, in order to achieve good conversions and yields, the catalyst must be added in stoichiometric amounts.
The reaction in aliphatic alcohols having from 12 to 18 carbon atoms that is described in EP 0 387 820 likewise permits oxidation at temperatures below the flash point of the organic solvent, but the process is not very attractive commercially since the reaction and the isolation of the 2,3,5-trimethyl-p-benzoquinone are very complicated owing to the relatively high melting points and boiling points of the alcohols.
An object of the present invention is to provide a novel process for the preparation of 2,3,5-trimethyl-benzoquinone in order to solve the, in some cases, considerable disadvantages described in the prior art as regards the cost of the materials used, the outlay involved in working up and, not least, regarding safety aspects, which prevent conversion on a commercial scale.
More particularly, an object of the present invention is, especially, to meet the following requirements of the process:
a.) Use of a catalyst system consisting of inexpensive materials which are freely available on the market and which generate the active catalyst species in situ under the given reaction conditions, in contrast to the catalysts described hitherto, which in some cases must be prepared in separate process steps before the actual oxidation reaction or are used up during the reaction. PA1 b.) Use of a catalyst system which is highly active and at the same time has a long life and which, after the reaction, can be recycled and used again repeatedly without special measures having to be taken. PA1 c.) Use of a reaction system consisting of different phases which are immiscible at room temperature, one phase containing the catalyst in dissolved or suspended form and a further phase containing the substrate and product formed during the reaction in dissolved form, which allows the substrate/product phase on the one hand and the catalyst phase on the other hand to be separated after the reaction and accordingly enables the product to be isolated in a simple manner and in a high yield and allows the catalyst phase to be recycled at low cost.